Saturday, October 16, 2010

6v Sealed Lead Acid Battery Charger (UC3906 based)

One of the last remaining hurdles of the chicken coop door projects is reliable off-grid power. 12v is the norm, but the motor is 6v. Wanting to learn more about chargers, power supplies, etc., I chose to build a battery charger that would keep a 6v 7Ah Sealed Lead Acid battery topped up and able to supply the motor with the 4-5A necessary for hoisting the door. In effect now, I've got my 6v battery-backed power supply, and a battery charger to boot. A mini-UPS, if you will.

This is what I need to keep charged:

The charger is based on a Unitrode (now part of TI) UC3906 (UC2906 in my case) SLA battery charge controller. The controller can operate in a couple of different modes, with regard to current vs. voltage sensing and state-change. You can find all the docs at: TI's UC3906 page. In addition, this page helped me along considerably. You'll note that I inverted the bulk/!float LED with the op-amp.

The charger is set at 1.2A bulk charging by a series of 2-watt low-ohm resistors. A series of resistor voltage dividers define the voltage parameters of the charger. The LM358D Dual-Comparator serves as a switch for two of the status LEDs, "Bulk", (yellow), and "Over-charge" (red) modes. The third LED, (green), is "Power" (or "Float-Mode").

Depending on the two or three states, depending on which mode you run it it, the charger will either follow voltage, or current. On power-up, OR if the battery voltage falls below 6.21v, the charger will "top off" the battery by seeking 7.15v and ending that mode when current falls to 200mA or so this current is set in part of the 2-watt series resistor equation.

The heatsink makes a HUGE difference. I used a smaller heatsink while prototyping, and, at full-current, over-charge mode, it ran at just under 200-degrees F. This thing runs at 97-degrees F. Big difference!

Here is the old heat-sink:

The heatsink donor board, a RE-PC score:

Would it be a hack without doublestick tape?

Even drilling a perimeter couldn't save me from at least one case crack, but hey, it's a hack:

The series of divider resisitors.. I was able to compensate for the 5% resistors by pairing them in series if they didn't measure out close enough to the target voltage. I found another person's UC3906 calculator spreadsheet, but I couldn't get the right values. I ended up creating my own Excel spreadsheet that represented the equations to set resistor values. They worked great. This thing tracks right at 6.90v.

"If a maintenance-free lead acid battery could discharge and charge with perfect electrochemical efficiency, there would be no emission of hydrogen or oxygen gas, just the quiet conversion of the plate material, one of lead and the other of lead oxide, both to lead sulfate while the sulfuric acid electrolyte changed to water while discharging; and the reversal of this process during charging.

The acid should be diluted with an excess of water to give a 5 to 10-folddilution. The liquid should then be neutralized by mixing with an equal volumeof agricultural lime, or other alkaline material containing lead-acid batterycalcium carbonate, orsodium hydroxide. Use ofhydrated lime has the advantage of lower cost per unit of neutralizing capacitywhen compared to sodium hydroxide.